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Are we at peak speed efficiency for jet airliners at Mach 0.85?
The cruising speed of large jet airliners has not increased in the past four decades. The 747 cruised at Mach 0.85 and the new Dreamliner 787 also cruises at Mach 0.85 even though it was designed 40 years later (1969->2009) and uses advanced, composite materials and more powerful and efficient engines.  Meanwhile, faster airliner designs have been shelved, most notably the Sonic Cruiser (Mach 0.98).
This seems to indicate that as long as we value fuel efficiency over travel time, that we have reached peak speed efficiency at Mach 0.85 and that the only difference will be the comfort (or discomfort) of travel at that speed.
Are there fixed physical limitations at Mach 0.85 that make this the most efficient speed for jet airliners? Does this mean that even with advances in materials science and jet engine technology, we will not see significant transoceanic speed increases in the foreseeable future - so long as fuel efficiency is primary? Will airline travel forty years from now -- 2054 -- still be at Mach 0.85?

Saving 10 minutes on a flight isn't that worthwhile when we're still spending hours waiting in airports. And it would in fact be a cost saving to abolish the security theater there.

That is why I indicated transoceanic flights in my question. There, a modest increase in speed could knock off 2 hours on  transpacific flights (NY->Singapore, for example). And taking a train or a bus wouldn't quite work as an alternative. :-)

Short answer: Increasing fuel prices would have driven down the most economical speed, but advances in aerodynamics have compensated for that and the maximum cruise Mach number is still at Mach 0.85. But there is a much simpler reason why this Mach 0.85 number seems so immutable.
Please note that we are talking about the maximum cruise Mach number; all airliners will fly more economical at slightly lower Mach numbers between 0.78 and 0.82.
Now we need to define efficiency. For airliners this is haul capacity per time, and can be expressed by the product of payload, range and speed. When none of the three constituents can be changed without making the product more costly to achieve, you have reached the point of best efficiency.
Engine Technology
The biggest change in the last 50 years was in engine technology. Bypass ratios have increased to values close to 10, the turbine inlet temperature is now 300°C higher than in the early jet age, and electronic control has improved especially the off-design performance of engines. This has made it less attractive to increase flight speed, so the optimum cruise Mach would be lower if no other effects had influenced it.
Another effect is the lower fuel consumption. Early designs had to use a high wing chord to pack enough fuel volume for transoceanic flights. The much lower consumption of modern engines has given them ranges which are more than what is needed to reach every point on earth directly. This made it possible to reduce wing area, thickness and mass, and to save some friction drag, helping fuel economy again. The lower thickness helps to make higher cruise Mach numbers possible.
Aerodynamics
Aerodynamics is the second factor. With the A310 supercritical airfoils were introduced, and they shifted the operating Mach number up because now a limited region of supersonic flow on the wing could be tolerated without a significant rise in drag. However, the benefit from that was used to reduce wing sweep, increase airfoil thickness and to attach bigger fuselages to those wings. In the end, the newer airplanes had the same cruise Mach numbers as their predecessors. Something made Mach 0.85 too attractive to abandon.
A second effect of supercritical airfoils is their blunter nose, which helps to reach a higher maximum lift coefficient. This also contributed to reducing wing chord, because now the wing area could be made smaller for the same landing speed.
The Real Reason
Every new airliner needs to compete with the older models. On fuel economy this is no problem, but speed is also important. A higher cruise Mach number allows  shorter times for connections. Now you have to know that the electronic reservation systems would list connections sorted by flight time, shortest first. When a travel agent wanted to book a flight, he or she would rarely look beyond the first screen and pick one of the first flights listed. If a new type made the flight slip down in the listings, it was a non-seller. This is even true in the times of online bookings: The airlines don't make much profit from the bargain hunters in Economy, their target are the business people flying in Business or First Class. And those still order mostly through travel agencies, so the rules have not changed. This is why that Mach 0.85 speed seems to be so firmly set; flying any faster would drive up fuel consumption disproportionally, and even the newest jets will mostly be flown around Mach 0.82 to Mach 0.84.
Making this speed possible is still a struggle for the design engineers, especially when you consider that larger aircraft would need thicker wings for structural efficiency, but need to be fitted with wings thinner than optimal to let marketing claim a cruise speed of Mach 0.85.

In order to actually fly to any point on Earth in a single flight, current airliners would need to be flying well below their maximum payload, though, right? For instance, the 777-200LR that currently holds the record (having actually flown more than half the circumference of Earth) was carrying only 27 passengers, as opposed to its normal 300. Its range at max payload is 'only' 10,793 miles (about 1,700 miles short of half the circumference of Earth.)

@reirab: Yes, right. But in effect all jet transports of the last decade are long-range transports. The short-range category has become extinct.

Well, there are tons of regional jets, but, beyond those, you're right.

Regional jets are 'jets' in name only -- they're turboprops. :-)

Fascinating answer, @PeterKämpf -- especially the 'real answer' which is not what I had thought would be a controlling parameter. I'll wait (to be polite) for other responses but I think your answer (as always) is the perfect response.

@RoboKaren: No, reirab is right. I was thinking of A320 sized jets and above. He thinks of CL-600 and ERJ 145, and their range is mostly still below transoceanic.

You might also consider that flight time is not just the time spent cruising at Mach 0.whatever, it's taxi, takeoff, climb, descend, land, taxi some more - and perhaps (I'm not sure if this is included in flight time, but it should be) the time to get those pesky passengers on and off the plane.  A small increase in cruise speed doesn't translate to a proportionate decrease in flight time.

@jamesqf: This is especially true for short hops, so regional jets are content to operate at Mach 0.78, where their operating cost are lower. But on transoceanic flights the shorter flight time will make enough of a difference to strive for Mach 0.85 as the maximum cruise speed.

@RoboKaren Yes, Peter is right, I was referring to jets more like the CRJ200/700/900 or the Embraer E-Jet family, not turboprops. At least here in the U.S., turboprops have all but disappeared from airline service, but regional jets probably come close to outnumbering B737/A320 class aircraft in service here. They primarily connect small-market airports to hubs, though sometimes they're used to medium-market airports, too, in order to increase daily flight frequency on a given route.

@reirab - Ah, I see. I live near a regional airport where we only have turboprops for service, so my view of regional services is very skewed. :-)

@Peter Kämpf: I was actually thinking more of the difference between mach 0.85 and say a marginal increase like 0.88-0.9 on a long-distance flight.  For instance, on a 5000 mile flight, the difference between cruising at Mach 0.9 vs 0.85 is only about 24 minutes (per Google's calculator).

@jamesqf: To go from Mach 0.85 to Mach 0.9 is a very steep uphill struggle for the engineers. Efficiency tanks on that way. Mach 0.78 - 0.82 is much more realistic.

Efficient operation also considers maintenance requirements.  Going faster than long range cruise uses more fuel, but also reduces how often maintenance must be performed. This offsets (or in some cases exceeds) the additional fuel expense.

@Lnafziger: The development effort and the increased thrust demands for flying at Mach 0.92 cost so much more that you can pay for a lot of maintenance. Even flying at Mach 0.85 is more a peeing competition than anything else, and the competition about ever faster (but still subsonic) business jets is exclusively for the egos of the owners. Your cost savings can only be had if you look at direct cost exclusively.

Absolutely, which is why I said compared to long range cruise.  I was referring directly to your third paragraph where you left maintenance out of the equation, even though it is directly affected by the speed at which you fly. Some maintenance is a direct cost (i.e. the $1,000,000 engine overhaul that is due every 4,000 hours is, especially if on an MSP progeam).

@Lnafziger: Now I got it. Yes, including maintenance will shift the optimum upwards for a given plane. I was more thinking of a new design, where the full life cost should be minimized. There maintenance is already a part of the picture, and the haul capacity per time drives speed up. If you minimize cost per flight for a given plane, you should indeed fly faster than long range cruise (which is normally quite a bit below Mach 0.85).

It's not anything inherent about Mach 0.85; it's Mach 1 that's the problem. The reason airliners are usually somewhere between Mach 0.8 and Mach 0.9 is because of the critical Mach number of the airframe in question. When the relative airspeed of the entire aircraft is at the critical Mach number, there will be some part of the airframe where the airflow actually reaches Mach 1. This is due to air accelerating around the airframe as the airframe moves through the air. The critical Mach number will be different for different airframes, but it's usually the limiting factor for the cruising speed of airliners. This question explains more about the critical Mach number.

Perhaps more to the point than the critical mach numbers (upper and lower) is the Drag Divergence Mach Number.  More reading here...

More or less.
The problem is the "critical Mach number" of the wing planform. Most wings have a curved upper surface, which forces the air moving over that side of the wing to move faster, lowering its pressure, producing lift. At a forward airspeed of the entire airfoil near but less than Mach 1, the airflow over the top of the airfoil exceeds Mach 1 as it passes over the wing's leading edge, then slows again to subsonic speeds around the crest of the wing. This "transsonic" airflow increases drag on the aircraft at airspeeds between the critical Mach number and the speed at which all airflow over every surface of the aircraft is supersonic (when transsonic airflow is no longer a problem, but other leading-edge and wake drag forces come to dominate total drag).
Commercial airliners sometimes exceed the critical Mach number of their wings, producing a shockwave that is visible either as a mirage-like optical disturbance, or if conditions are just right, a vapor plume:

Fighter jets accelerating past Mach 1 commonly create "vapor cones" beginning at the transsonic border of their airflow, caused when the high variance in air pressure at this transsonic shockwave border forces water vapor to condense:

The critical Mach number is determined by the design of the planform. Wings with thicker cross-sections produce more lift for less area (and more induced drag from that lift) and generally also have a higher angle of attack (meaning, all other things being equal, a lower stall speed), but the tradeoff is a lower critical Mach number. The converse is true for a thinner cross-section; critical Mach number is increased, while lift, drag and critical AoA are reduced.

Where in your upper image is the vapor plume visible? Is it that thing on the right? Some arrow or circle would be nice.

@Giovanni I see what appears to be a "mirage-like optical disturbance" in the upper photo, right above the third rectangle from the left.  It gives the appearance of a strange discontinuity artifact in the image.

@Michael you're right about the disturbance, but I can't even figure out the picture. Is it the top side of a wing looking backwards?

@Eugene that's a great question. all the pictures I have taken out the window of a plane show a much more flat wing without any apparent edges or protrusions.  I had assumed that the upper/left was the back but now I'm not so sure, it kind of looks like it may be the front.


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Which flight maneuver is better for a takeoff emergency after liftoff, a tear drop (180° turn) or a 360° turn?
Let us say I got a time critical emergency right after the liftoff (like a fire, complete power loss) and I need do a return-to-runway and re-land maneuver. Which option is better, a 180-degree turn for a tailwind landing or a 360-degree turn for a headwind landing?

It probably depends strongly on whether you have any working engines left or not.  A fire in the cabin is different from engine-out.  But your question just groups these things together, and the answers all seem to be about the no-power case, where you can't power through 180 turn.

Ahem. A 360 is basically useless, as you end up exactly where you were, but just lower !

@kebs I think you can assume the OP means a 360 back to the other end of the strip.  LOL.

@RockPaperLz-MaskitorCasket Sure, got that. But I wanted to point out that the question was ill formulated. Technically, when you get ordered from ATC to "do a 360", that is... a 360 turn ! What he meant here (?) is just a "circuit", just a short one, to get back to runway.

We've all answered for Single Engine Piston (or lower, in the case of the glider answer!). Can you confirm that was what you were interested in the answer for - as multi/jet etc changes things quite considerably

Neither! The right course of action with engine failure or emergency after take off is land straight ahead within 30 degrees of straight.
If there is enough runway ahead of you, land on that. That is the first (and best) option.
180 and land is possible, I've seen it done a couple of times. If it goes badly, it goes REALLY badly. The likelihood of being able to glide an entire circuit (traffic) pattern is almost non-existent unless you're quite far down the downwind leg - you only have to practice glide approaches from the pattern to realise that.
The only time I would even attempt it if I had a partial loss of power and was able to maintain straight and level flight, then I might limp round the pattern and land as usual.
But I re-iterate, any significant emergency below circuit height, with significant loss of power or significant need to get back on the ground ASAP almost requires an emergency landing straight ahead.

Comments are not for extended discussion; this conversation has been moved to chat.

Some aviators have performed tests in various aircraft - see the video below, in short the results show that a 180 works for some aircraft, under some circumstances with some pilots. A 180 back to the runway landing in the opposite direction is more than 180° though.
The bottom line is however, it is better to land within 30° either side straight ahead.
And may I suggest, with modern technology, especially if your airfield is surrounded by urban sprawl, to study the after takeoff areas using your favorite satellite image app (mine is SASPlanet) and have predetermined areas in in your mind in case such an emergency arises.

In glider training I was told to prepare for line break, ie the line from the tow plane would break (or other mishaps). Up to a certain height, straight ahead within a limited arc. Select and plan for the fields to land on (different due to season and type of growth). Up to next height, 180 degrees. Above that height a full landing pattern. Of course, the actual height would be different depending on type of plane and other circumstances.
The main point was (is): prepare for problems and create plans -- once there follow the plan and Aviate, Navigate, Communicate in that order.
When training for powered flight from a municipal airport, my teacher did show alternative emergancy landing sites causing as little problem  as possible for me and innocent bystanders. Better to aim for a small lake than a large office building.

To be fair, a glider has a much better glide ratio by nature, so a turn back to the field should be much less of an issue.

@MD88Fan: not really less of an issue. Eagerly, perhaps afraid, and doing a 180 when low on energy seems to be a sure recipe for "uncontrolled" flight into ground. It is quite easy to enter spin controls when trying to "heave" the plane back to the field.

@MD88Fan, that basically just affects the altitude above which it becomes an option.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Is this blue one called 'threshold? Or 'runway threshold bar?'
Is this blue one called 'threshold? Or 'runway threshold bar?' in FAA?

AIM 2-3-3-j refers to this painted stripe as a “Runway Threshold Bar” In the paragraph heading, and then refers to it simply as “a threshold bar” in the paragraph.  Quoting directly from the AIM:
j. Runway Threshold Bar.  A threshold bar delineates the beginning of the runway that is available for landing when the threshold has been relocated or displaced.

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Why is a part from a Boeing 777 being analysed by Airbus?
Since MH370 was a Boeing 777, why are they taking the suspected wing flaperon to Toulouse, France?
Are some spare parts made by Airbus?

Is it confirmed that the wing was from MH370?

@Firee Even the accident investigators with normally cautious press interaction are not really doing much to cut down on speculation...

@MikeFoxtrot: Could it be a discarded wing of an old 777 tossed into the sea, or fell from a ship

@Firee "old" 777 ?!?

Why is part from B777 being analysed by Airbus Who said that it is?

@Federico: I meant, the 777 in service today which might be older, and might have required a change of flaperon

@Simon: I think MikeFoxtrot has mentioned a source in his answer below

@Firee I've read the whole thing and I must be missing something.  Nowhere can I see mention of Airbus being involved.

I think some people are just confused since they associate 'Toulouse' very strongly with Airbus...

@Firee It is now.

I've put the term flaperon to Goole Translate and it said it was not English but Esperanto. Just saying.

@Firee it was confirmed that the part was from a 777, and since no 777's other than MH370 have gone missing, it would have to be a pretty interesting circumstance to not be from MH370.

Downvoted for false and baseless premise! Toulouse is a city, not a single building for a single company. You know what assuming does...

Why are they taking the wing to Toulouse, FR?

Because the part was found on the island of Réunion, which is French territory.

Why is a part from a B777 being analysed by Airbus?

It is not. Airbus is not the only company/institution to have plants/facilities in Toulouse: The BEA (France's equivalent to the NTSB) also has facilities there.

There is a lot of aerospace activity in Toulouse. The French space agency, CNES, operates out of Toulouse - Toulouse Space Center.

To extend a bit on Federico's answer. As I understand, BEA, the French accident authority, have some office in Toulouse while the facility itself where it will be looked at is military.

France's BEA crash investigation agency will examine the wing flap at the Direction Generale de l'Armement compound near Toulouse. Source

Also from the BEA website, under the heading ICAO:

In particular, States must start an investigation in case of a civil aviation accident or serious incident on their territory. It is specified that "The sole objective of the investigation of an accident or incident shall be the prevention of accidents and incidents" and that "It is not the purpose of this activity to apportion blame or liability".
Close association between the representatives of the States of Registry, Operation, Design and Manufacture of the aircraft is organised. This brings to the State that is leading the investigation useful skills and information and allows for feedback that is as fast and complete as possible in terms of safety. Source

Which explains their responsibility in investigating the part. I don't think there are too many facilities with extensive knowledge of forensic engineering on impact-damaged (composite) parts (Even analysis of damaged Black Boxes is often 'outsourced') so they might even be happy to have the large BEA look at it.

The French are not unfamiliar with investigating aircraft accidents and have participated in a few substantial ones in the last few years. I discovered presumably the same folks were involved in forensic engineering work of the AF447 remains in 2011, which also crashed into the ocean.

French investigation agency Bureau d'Enquetes et d'Analyses said the vessel would transfer to Bayonne, a port on the south-western coast, and parts from flight AF447 would be moved to a hangar in Toulouse.
The wreckage would be examined by the French defence ministry's technical aeronautical division. The recovery effort also retrieved several victims of the crash and these will be taken for forensic assessment.

Footnote: Since accidents with unclear events (or presenting controversial claims like crashing the plane) such as this one which can be a hot potato it is sometimes even desired to have a independent third party look at the stuff.

It is also perhaps worth noting that Boeings have long been used by French companies (notably B737, 747, 777 and now 787 by Air France), in much the same way Airbus aircraft have been used in the States. I am also sure many Airbus engineers are current on their competitor's offerings, if only to better counter them. So I wouldn't be surprised there is actually quite a lot of knowledge about Boeings in the aircraft sector in Toulouse. BEA will have a large pool of potential consultants to draw on if necessary.

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What is the difference between centre of pressure, aerodynamic centre and neutral point?
I have just started learning some aerospace concepts, and I am not able to understand the difference between the three terms centre of pressure, aerodynamic centre and neutral point. What are their functional differences?
What would be an example that distinguishes these three?

In this sentence: As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of gravity affects the stability of the aircraft. - surely there is a misprint or absent-minded word?? Because it should read: As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of PRESSURE affects the stability of the aircraft.

Any body moving in a fluid experiences pressure forces over its surface. The concepts of center of pressure, aerodynamics center and neutral point are useful in understanding the effects of these forces. Let's take an airfoil moving in air with subsonic flow attached to the body.
Center of Pressure The center of pressure is the point where the total sum of a pressure field acts on a body. In aerospace, this is the point on the airfoil (or wing) where the resultant vector (of lift and drag) acts.

Source: avstop.com
As the airfoil angle of attack changes, the pressure field changes. Due to this, the center of pressure changes with variation in the angle of attack. In the airplane's normal range of flight attitudes, if the angle of attack is increased, the center of pressure moves forward; and if decreased, it moves rearward.

Source: www.flightlearnings.com
As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of pressure affects the  stability of the aircraft.
Aerodynamic Center The resultant (or the pressure forces) also cause a moment on the airfoil. As the angle of attack increases, the pitching moment at a point (for example, the center of gravity) also changes. However, the pitching moment remains constant at a particular point, which is called the aerodynamic center.

Source: www.theairlinepilots.com
For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point.
Thus the aerodynamic center does not change with variation in angle of attack. Due to this, the aerodynamic center, rather than the center of pressure is used in the analysis of longitudinal stability.
Neutral Point Consider the whole aircraft. The lift of the wing acting through the center of pressure is in front of the center of gravity of the aircraft. This causes a destabilizing motion (increase in lift to increase in angle of attack causes a nose up moment, further increasing angle of attack). This is counteracted by the moment produced by the lift of the horizontal stabilizer (which is a small wing), acting behind the center of gravity.

"AirStability" by User_A1 - SVG derivative of Public Domain work by G J Vigurs. Licensed under GFDL via Commons.
As the center of gravity is moved forward, the stability of the aircraft increases (as the main wing lift arm is reduced) and the aircraft is statically stable.

Source: quest.arc.nasa.gov
As the center of gravity is moved aft, the main wing lift moment arm increases and the aircraft stability decreases. The aircraft is said to be statically unstable.

Source: quest.arc.nasa.gov
As the aircraft is stable when center of gravity is in the nose and unstable when center of gravity is in the tail, there is a position in the middle where the aircraft is neither stable nor unstable i.e the stability is neutral. This point is called the neutral point. The neutral point is fixed for a particular aerodynamic configuration of the aircraft.

Source: quest.arc.nasa.gov
Thus,

Center of pressure of an aircraft is the point where the Lift acts.
Aerodynamic center is the point in the wing where the pitching moments are constant.
The neutral point is where the center of gravity of the aircraft is neutrally stable

Notice how a right sized (area) Hstab will help static stability a great deal with very little drag penalty. Ideally it would be near 0 angle of attack at cruise, holding the wing at its optimum AOA.  Simple flat plate, low aspect Hstabs are common for a good reason.

for neutral stability the aircraft's moment slope should be 0. In other words, the pitching moment is constant. The neutral point of an aircraft is its aerodynamic center. Of course, this not the aerodynamic center of its wing.

Good answer but perhaps a slight improvement is possible.  "However, the pitching moment remains constant at a particular point, which is called the aerodynamic center."-- is this for a given airspeed?  Would it be more accurate to say the pitching moment COEFFICIENT remains constant if we treat the lift vector as acting at the aerodynamic center?

Center of pressure: the net force of air on a body/surface/wing. The center of pressure changes as an aircraft maneuvers because different parts of the aircraft stick into the airflow as the angle of attack changes. Notice how the center of pressure moves forward as the angle of attack increases (the airflow is right-to-left):

(source: prepware.com)
Aerodynamic center: the location at which the moment does not change regardless of the angle of attack. Wings tend to produce a rotational force due to the way they push air around, and this is the location at which the rotational force stays constant at any angle of attack. It is often easier to consider the aerodynamic center for where you want to place your wings and mass because its location is constant, unlike the center of pressure. For now, think of Mtrim as equivalent to the rotational force the elevators must impart in order to achieve level flight.

(source: charlesriverrc.org)
Neutral Point: the position of center of mass where the aircraft would be neutrally stable. If you put your center of mass behind (aft) of this point, you will get an unstable aircraft that will increasingly turn with any maneuver the pilot makes. At low speeds, this may be tolerable, but computers are needed to fly unstable aircraft at higher speeds.

(source: rchomepage.com)
tl;dr:

CoP = where the wings push
AC = where the wings rotate
NP = where the center of mass should be to make an aircraft that does not tend to pitch up or down

Is it accurate to also describe the neutral point as the centre of the couple between the CoG and the AC?

@Simon That may not be the case. In tailless aircraft, the CoP and NP coincide, so a modified airfoil is often used to counter the AC.

The first sketch is lousy - the center of pressure is generally too far back and should be close to the quarter chord at the high angle of attack. The real Kurt Tank would have noticed that immediately!

@PeterKämpf Ah, you see I lost my high-quality drawing tools in the war. Notice that the 3 degree angle and 9 degree one look suspiciously close!

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Why might a passenger jet fly at altitudes in a strong headwind when the winds are more favorable at a different altitude?
I was a passenger on an east to west flight recently. During the flight I was playing with Garmin Pilot's new "Flight profile" feature that nicely shows the wind speeds at various altitudes, and I noticed that we were flying against an 80+ knot headwind at FL320, but if we'd dropped to FL220 we'd shave about 40 knots of headwind. That seems pretty significant. A 1,000 NM flight with 355 knot ground speed (which we were cruising at) would save 17 minutes of cruise time with 40 knots less headwind. That's a 10% increase in efficiency.
So the question is, did we stay (or even plan in the first place) to fly at FL 320 the entire cruise portion of the flight simply due to ATC needing to keep us there for traffic routing purposes,
Is there an even larger increase in fuel efficiency between FL320 and FL220 than 10%,
or is the difference negligible enough that it's not worth asking for a different altitude from ATC,
or some other factor not considered here?

winds are quite volatile so you can't rely on them staying the same for several hours

Pretty good answers below working through the math, but it must be stressed that the airlines optimize for minimum fuel cost much more strongly than for minimum flight time. While there are some other costs that go up with flight time (crew pay, aircraft maintenance), fuel is a big, big component and they fly to make money. I also find it useful to keep in mind the very approximate rule of thumb that true airspeed increases around 2% per 1,000 feet of altitude. (The real equation is far from linear so this a crude estimate, but useful.)

You assessment is flawed in that it does not take different air density in account.  You state the following conditions:

FL320
80 kt headwind
355 kt groundspeed
1000 nm distance

and you are proposing descending to the following conditions:

FL220
40 kt headwind

At 32,000 ft, standard conditions are around 275 hPa, 225 K and density of 0.43 kg m$^{-3}$.  If your 80 kt wind is a pure headwind, a 355 kt groundspeed translates into a 435 kt TAS. If we correct this TAS for the ambient conditions we get an indicated airspeed of about 255 KIAS.  This is the airspeed that really matters because it is what the airplane experiences as it interacts with the atmosphere.
If we descend to FL220, where standard conditions are 430 hPa, 244 K and 0.61 m$^{-3}$, an airspeed of 255 KIAS yields a TAS of 363 kt TAS.  If we have a headwind of 40 its, this yields a groundspeed of 323 kt.  This is about 30 kt slower than you were flying at FL320.  A flight of 1000 nm will take about 15 minutes longer to complete.
However, it is most likely possible that the airplane you were in is capable of flying faster than 255 KIAS at FL220 and in fact is probably capable of somewhere around 310 KIAS.  Flying at 310 KIAS at FL220 will have true airspeed or 440 KTAS and with the 40 kt headwind a ground speed of 400 kt. This will get you to your destination faster, but at the expense of a much higher fuel burn.
It is also likely that you could have been flying faster at FL320 (not sure what max mach number would have been for you), but also at the expense of burning more fuel.  If the airplane wasn't dispatched for the additional fuel burn to go faster at FL220, then after descending you would probably not have enough fuel to go fast enough to make up the difference in true airspeeds.

Great answer. I don't agree with your first statement that my assessment was flawed, as one of the options I presented indirectly acknowledged that density altitude would be a factor, but otherwise, very instructive. I'm a little surprised by the magnitude of difference the density altitude makes in this example, without doing the math, it didn't seem like it would be that big.

@GregTaylor I'm getting .74 mach. Perry slow for an airliner. What kind of plane were you in?

It was an MD-80, technically S80.


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Can I log simulated instrument time in a complex plane without having a complex endorsement?
My safety pilot is endorsed for complex, and will be PIC for the entire flight. I would also log PIC for the time I spend under the hood.
I will eventually get my complex endorsement, but insurance is much cheaper if I have time in type, so in the long run it will be cheaper for me to build time in type and work towards my instrument rating at the same time.

Yes. You can, as long as you have the appropriate rating and are the sole manipulator of controls. From a FAA response to a similar question:

... a private pilot may log pilot-in-command time, in a complex or high performance airplane, for those portions of the flight when he or she is
  the sole manipulator of the controls because the aircraft being operated is single-engine land and the private pilot holds a single-engine land rating.
... 14 CFR section 61.31 requires a person to have an endorsement from an authorized instructor before he or she may act as pilot in command of certain aircraft (a complex airplane, a high performance airplane, a pressurized airplane capable of operating at high altitudes, or a tailwheel airplane). These endorsements are not required to log pilot-in-command time under 14 CFR
  section 61.51(e). (emphasis mine)

Excerpt from §61.51 Pilot logbooks:

(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-
(iv) When the pilot performs the duties of pilot in command while under the supervision of a qualified pilot in command provided—
(A) The pilot performing the duties of pilot in command holds a commercial or airline transport pilot certificate and aircraft rating that is appropriate to the category and class of aircraft being flown, if a class rating is appropriate

Yes.
You can log actual and simulated instrument time while being the sole manipulator of the controls without the complex endorsement.
See FAR 61.51.

A link to the relevant article may be helpful as well as a quote of the relevant part.


text:
How can I solve this question using the 1 in 60 rule?
What is the explanation for the question below in detail?

A pilot is heading east and decides to fly to an alternate airfield which is 4NM south and currently 30NM from his present position. What is the new track to arrive at the alternate airfield?

A - TRK 008°

B - TRK 098°

C - TRK 100°

Answer: B

I can't understand how the 1 in 60 rule would be applied to solve this here.


text:
Will all turbine blades stop moving in the event of a emergency shutdown?
In the event of an emergency shutdown of an engine (while in the air),

are there systems which prevent movement of turbines (brakes) in case of internal damage?

Or will that cause aerodynamic drag and therefore not practical?

Clarification:
I am thinking of a case where debris within the turbine will cause more damage (e.g., severing of hydraulic lines, electrical lines, etc.) if the turbine continues to spin. I know that if debris was to enter, the damage caused would already be catastrophic, but I’m curious if such emergency braking systems are in place and whether or not they are applied automatically.

The idea I had in mind is if a fire broke out; presumably in the air; stopping everything ( immediately ) could (excluding the obvious, it needs to be running since it's in the air)  the equipment could I guess get damaged? (I am not sure though) It would make sense if they would slowly shutdown instead of a total immediate stop; at least to me that is; I will follow this post; good question!

If a fire breaks out, the fuel line to the engine will be shut down and the engine will stop to rotate "per inertia". To stop it with a brake would request a massime brake due to the huge inertia of the spinning parts and it would simply be a waste of weight and a source of an (extremely dangerous) failure case. Brakes are applied for example to helicopter's rotors but only to avoid their spinning on ground due to the wind and they can be applied only when the rotor has come to an almost complete stop. If you want, I could elaborate this comment in an answer putting some numbers in it.

No. Assuming the rotating bits are undamaged, both the engine core and the fan section will windmill in the airflow. The benefit of stopping the engine spools is not worth the complexity, cost, weight and failure modes introduced.
In any case, you want the engine windmilling because that's one method to get it re-lit if you're above the maximum altitude for using the APU for starting, and for some reason you can't start by cross bleed (like say, double engine flameout at high altitude). There will some minimum speed to need to achieve to get the core spinning fast enough for a windmill relight.
If the engine flames out while at high power, "core lock" can happen, where the engine case shrinks faster than the compressor/turbine causing the blades to brush the walls of the case. If you get below some speed, it can allow the core to stop completely until the spool shrinks enough to release the blades. You may not be able to restart the engine even off the starter, air turbine starters making very little torque, until that happens.
Windmilling tendency depends on the strength of the ram air flow through the engine core, which depends on the compressor inlet configuration and the service loads of things like pumps driven from the core spool. With most engines, the engine will windmill at just about any speed, but on others, the core will stop at some low speed, and you will need to maintain some margin above that to keep N2 from going to zero to guarantee relight capability.
So, overall, a windmilling engine is desirable.

As promised, here a couple of numbers (very simple equations, don't be scared).

are there systems which prevent movement of turbines (brakes).

A brake basically converts the kinetic energy possessed by whatever it brakes into heat.
In this specific case we want to brake the spinning part of a turbofan. The kinetic energy of something rotating is given by a formula very similar to the well known $½mV²$:
$E_k=½I\omega²$
where $\omega$ is the rotating speed and $I$ is the moment of inertia of body; $I$ depends not only on the mass of the body but also on its distribution around the rotating axis.
Unfortunately I couldn't find any value of $I$ for a real turbofan/jet (I suppose it's a confidential information), so we make a ballpark estimation of the rotating masses: going from the inlet to the outlet, we normally find the fan, several compressor stages and several turbine stages. Even if they have a more or less complicated shape, from a moment of inertia point of view each of those stages is basically a rotating disk; and when they are considered all together, they become a cylinder (or sort of). For a cylinder of radius $r$ and mass $m$, the moment of inertia is simply $I=½mr²$ and its  kinetic energy is therefore:
$E_k=¼mr²\omega²$
Now, all of the stages previously introduced have obviously a different radius and at least two different rotating speeds so we just use an average value for them: we are not going to get a precise value for the energy but at least its order of magnitude should be fine.
Let's use then a modern PW1100, as found for example on the A320, making the following assumptions:

We consider the mass $m$ of the rotating parts as ⅓ of the total mass; ⅔ of non-rotating mass should be fine considering that for each rotating stage there's a stator stage plus the combustion chamber plus all the pipes, the outer case and other structural components; $m=2850/3=950kg$.
For $r$ we should take into account that the compressor stages get smaller, the turbine stages get bigger and that there's a big fan of 1m radius in front of everything; we just use an average value of some 0.3m.
the low-pressure stages rotate at 10'000rpm and the high pressure stages rotate at 20'000rpm; we just use the averaged value (of the squared, since in the equation for the $E_k$, $\omega$ is "²") i.e. $\omega=16000rpm=1650rad/s$.

Finally:
$E_k=¼ \times 950 \times  0.3² \times 1650²=60MJ$
Again, this is a very rough ballpark estimate, so the $6$ in front of "$0MJ$" is wrong but for sure we can say that the rotating parts of a modern turbofan posses a kinetic energy of some tens of millions of Joule.
Is it big? Well, at landing, the whole A320 has a kinetic energy of $½mV²=½\times 64000 \times 80²=205MJ$. Some of this energy is dissipated by the friction of the wheels with ground and some by the aerodynamic drag but most of it (some 80%) transforms in heat in the brakes. Comparing this value with our rough estimation, we've then obtained that the kinetic energy of a turbofan is more or less ¼ (let's round it a bit) of the kinetic energy of the whole landing airplane it is mounted on.
Since the A320 has four braked wheels, a brake on the shaft of a turbofan, would look something like this (source)

Being a part which would be used maybe once every some billions of flight hours, it would be pretty big, heavy and actually dangerous in case of failure.


text:
How can business jets cruise so fast?
How can business jets cruise so much faster than airliners in the transonic regime?
If you look at the cruise speed of many business jets, it will be somewhere in the Mach 0.90 to Mach 0.94 area. This is very fast and in theory would consume much more fuel than if they cruised at around Mach 0.85 which is around how fast most airliners cruise.

Based on my rudimentary knowledge of these things, I feel like Mach .94 doesn't really make sense for anyone. Wouldn't that put it firmly in the trans-sonic flight scheme, where the plane would have to be built for supersonic flight anyway? And nudging it into supersonic territory would also be more efficient?

@BlackThorn indeed. Each plane is built for a certain purpose. In most business and airliners speed is not one of them. But as answer stated, business jets try to save time and so are a little fast. So technically speed is one of its purposes but is not the #1 priority.

@BlackThorn The higher Mach number gives you bragging rights later at the golf course or the country club. Invaluable for some!

@BlackThorn: Most jetliners are already well into the transonic regime at M0.85; bizjets are just somewhat further in than jetliners.

Business jets are built to save time for their passengers, who are often quite wealthy.  Therefore, the design is based around favoring speed over efficiency.
Commercial airliners are built to make a profit for the airline, by moving passengers faster than the alternatives (cars, trains, and buses) and keeping a useful schedule with connections to other flights. Therefore, the design is based around favoring economy; saving money on fuel to maximize profit.
Many airliners can cruise faster, but at the expense of more fuel burned and tickets that are less affordable.
Many business jets can cruise slower and save money, but the important, wealthy passengers often wish to make the trip as fast as practical for their next business meeting (or tee-time).

Makes sense but how much useful time could actually be saved by going this much faster?

Interested to know how much fuel is saved when a typical business jet travels ~0.1 mach slower (0.94 -> 0.85). For, say, London to NY, we could then calculate the cost saving, and then divide it by the cost of the flight (as a percentage of the total flight cost, it might only save a few percent).

The other major time-saving element of private jets is that you don't have to deal with all that drudgery at the airport.  The vast majority of time for taking a commercial flight is spent going through the security lines, waiting for hundreds of passengers to board, loading up the luggage, etc., and then again in reverse at your destination.  With a private jet, you can just drive up to the plane and board immediately, and take off within minutes.  The actual flying part of the trip doesn't have to be that much faster when you cut out all the waiting before and after.

cruise speed of many business jets... will be somewhere in the Mach 0.90 to Mach 0.94 area.

All of the Cessna Citation series of business jets have a maximum speed (in Mach) of 0.8.

All of the Dassault Falcon series of business jets have a maximum speed of 0.85 to 0.9 with a "long range" cruise speed of 0.8.

Bombardier lists 0.83 as the maximum Mach for its Challenger series.

So most of the current business jets actually fly at the same or even at a lower speed than any modern jetliner (Mach 0.85; 0.9 for the Dreamliner), no matter how rich or in how much of a hurry the VIP on board is (btw, this answer gives a very nice explanation about why Mach 0.85 is the sweet spot for jetliners).
That I know only the Citation X lists a higher maximum speed (Mach 0.93) together with the Gulfstream G650/G700/G800 (Mach 0.925). The former is anyway no more in production and the latter, according to Wikipedia, have a market share of some 13%. So those high Mach values are more of an exception than the rule: even the ultramodern Pilatus PC-24 and the HondaJet have a maximum speed comparable to all the other business jets. Yes, sure, the next generation of business jets is being advertised with incredible max speeds just slightly smaller than Mach 1 but then next to it there's printed a more plausible Mach 0.85 as cruise (real?) max speed, so this is more the marketing department at work and not physics.

How can business jets cruise so much faster than airliners in the transonic regime?

They don't. And they can't (again, except the Citation X or the Gulfstreams). To reduce transonic drag as much as possible, one or more of the following aerodynamic tricks has to be used:

the wing is swept of a couple of tens of degrees $\rightarrow$ many of those business jets (especially Citations and Falcons) have an almost straight wing for ease of manufacturing;
use of thin airfoils for the wing $\rightarrow$ according to this answer Cessna has used airfoils with a quite high 12 to 14% thickness; this gives higher max $C_l$ and fuel volume but at the expense of a higher drag;
area ruling $\rightarrow$ doesn't seem to have been implemented in any of those business jets and a straight wing definitely plays against it.

In terms of speed, the real gain of a business jet is actually the much shorter "rest of the flight other than cruise" phase: no need to get to the airport two hours before the flight, no queue at the check-in, no queue at the gate, no queue for boarding and so on. This saves a couple of hours per flight. Plus, obviously, one can fly whenever and wherever wanted (and without any stinky fellow passengers).

Also worth noting that even 0.93 isn't really that much faster than most airliners designed for long-distance flight. The 747-400's MMO is 0.92, for example. 777-300ER is 0.89. It's more the narrowbody aircraft (designed primarily for relatively short flights) that have the lower MMOs (0.82 for 737 and A320, for example.)

@reirab: correct, that 0. something Mach more doesn't change anything for a flight of one hour, that's why many regional airplane are more efficient turboprops.

Yep, though that's more regional. All of the major airlines in the U.S. (including the regional airlines that operate under contract for the major carriers on short/thin routes) have stopped using them, for example, with Alaska (the airline, not the state) retiring their last Dash 8s this year.

Just being able to avoid a connection would be a huge time saver, probably more than all the other items you mentioned.

The logistics for boarding a private jet is VERY fast. After being cleared to enter the airport (very quick and basic clearance), you can literally drive up to the plane, board, and take off. They don't care about scanning your bags because if you blow yourself up, it's your own plane.

By and large, long range airliners and long range business jets have very similar cruise speeds.  A 747 and 787 both have a Maximum Operating Mach Number (Mmo) of 0.90 but typically use a maximum cruise speed of Mach 0.85 for their flight profiles and Mach 0.8 for long range economy cruise.
The biggest and fastest luxury business Jets these days e.g. Gulfstream’s G500/600/650/700/800 and Bombardier’s Global Express series all have an Mmo of 0.925, but use a high speed cruise of Mach 0.9 with an economy cruise of Mach 0.85.  There has been very little efforts made on the part of these luxury bizjet companies to go faster than this. Even with Cessna raising the bar with its now defunct Gen II Citation X boasting an Mmo of 0.935, none of the other major bizjet players followed suit in the speed wars. The reasons I’m not clear on; it could be that M0.925 offers the best ability to wring out as much speed as possible while offering a reasonable safety envelope against Mach tuck or other undesirable high speed effects while maneuvering. All of the above listed aircraft have exceeded Mach 0.99 during flight testing.
Regional jets and intermediate range
airliners have limited themselves of an Mmo of 0.8-0.82 or so with cruise speeds of Mach 0.75-0.8. That might seem slow, but consider this: a Mach 0.75 cruise (430 KTAS) is 83 knots slower than a G650 in high speed cruise at Mach 0.9 (516 KTAS).  On a typical regional flight (ex. KLAX-KLAS), you will, at most have 12-15 mins in cruise prior to starting down on a STAR with its typical speed restrictions or those imposed by ATC. A Mach 0.9 cruise under these condition will, at best, shave about 2-3 mins off that flight; those gains could easily be lost if ATC requests you hold somewhere, how they sequence you for the approach, etc. Once below 30,000 ft they will typically restrict you to 280 KIAS or below, and even if they didn’t, you would probably maintain those slower speeds to fly the descent profile smoothly and get slowed up for the approach into the airport. Where the Mach 0.9 cruise can shine is long duration cruise.  For a flight from KLAX to Narita Intl., Tokyo (~4,900 NM), the Gulfstream does it in 9.5 hours @ M0.9 vs a 787 at 10.05 hours @ M0.85, assuming calm winds. This saves approx 33 minutes on the trip.
Where business aircraft always beat airliners is in terms of passenger comfort and convenience.  They are much more luxurious, quieter and with cabin pressures exceeding 10 psi, they definitely reduce fatigue during long-haul flights.  The ability to schedule the flight whenever you need to and from whatever airport best suits your mission is another major factor.  That alone can slash hours off your travel time, regardless of whether you made the trip at Mach 0.9 or not.

… and the biggest time saver for business jet users is their ability to fly from smaller regional airports. Travel to and from the airport takes less time as does getting a slot when you need it. The faster flight speed is just some icing on the cake.

Honestly, I'd say it's much more the ability to leave on whatever schedule from whatever airport (including ones without passenger terminals) and without dealing with security, baggage check, etc. than the passenger comfort in most cases. The passenger comfort on many biz jets isn't that great compared to a (far cheaper) business-class ticket on a major airline, where flat-bed seats are the standard nowadays for longer flights (and also some shorter ones, depending on route.)

Most commercial jets from after the 1950s (both jetliners and bizjets) lack a major tendency to mach-tuck at transonic speeds (this is how they're able to cruise at transonic speeds); first-generation Learjets do have such a tendency, but they're very much the exception.

Thrust-to-weight ratio
Business jets have more thrust relative to their mass for

better short field performance,
higher cruise ceiling, and
higher top speed
compared to large passenger jets.

The speed is just a welcome side effect of their ability to fly from smaller regional airports and to fly above most other traffic. The first two points are also favored by the lower wing loading of business jets, which, however, is rather detrimental to a higher airspeed. The combination of higher thrust loading and lower wing loading helps more than the high cruise speed to make travel times shorter:

Flying from many more airports means they can avoid the congestion of major hubs and the travel time to and from the airport will be much shorter for their passengers.
Flying higher allows them to take a direct route and frees them from the need to co-ordinate with scheduled traffic which would often incur holding times.

To prove my point, here are some figures (data taken from the respective Wikipedia page):

Type
static thrust
wing area
take-off mass
T/W
m/S

Learjet 35
2 x 15.6 kN
23.53 m²
8,300 kg
0.383
353 kg/m²

Citation X
2 × 31.28 kN
48.96 m²
16,601 kg
0.384
339 kg/m²

Gulfstream G-550
2 x 68.4 kN
105,63 m²
41,050 kg
0.34
389 kg/m²

Bombardier Global 5500
2 x 67,3 kN
94.8 m²
41,957 kg
0.327
443 kg/m²

Boeing 747-400
4 x 276.2 kN
541,2 m²
396,893 kg
0.284
733 kg/m²

Boeing 777-300
2 x 433 kN
436.8 m²
299,370 kg
0.292
685 kg/m²

Airbus A340-600
4 x 249 kN
439,4 m²
 368,000 kg
0.276
837 kg/m²

So their speed is indeed a consequence of the goal to make travel times as short as possible, but mostly indirectly. The largest time savings come from shorter travel times to and from the airport, followed by the ability to fly directly and at any time without waiting for a slot at a congested major airport. The high thrust gives business jets better short-field performance which allows them to fly from a much larger number of airfields and, since thrust is roughly proportional to air density, also allows them to fly in lower density air. The higher speed is just a side effect and helps to save a few minutes more.

For completeness I think it should be nice to consider the wing loading as well

@sophit Yes, lower wing loading is also a requirement for short field performance and high ceiling, but it is detrimental to high speed. The question is about high speed, so wing loading is not the right answer.

Business jets are able to cruise at over 50,000 feet, above all but the very worst weather.

economical cruise speeds are considerably less

More in the Mach 0.80 to 0.085 range.  But higher top speed sells.
Newer models, such as the  Bombardier Global 8000, have been tested near or exceeding Mach 1, and can be flown at speeds of over Mach 0.9 to save time.

Any that's been specifically tested exceeding Mach 1? There is a difference between 0.95 and 1.01.

@Therac read here about the Global 8000.  Mach 1.015 in testing.

I think most answers so far seem to be missing a key point. We all agree that the point of business jets is speed in terms of A to B, but that is not what Mach measures. If you could do Mach .8 at sea level, you would be doing 609mph. At 40,000 ft, .8 is only 528mph (Mach .9 685mph at sea level, 594mph at 40,000ft). Having less parasitic drag means being able to go faster and being able to go higher means they are yet more efficient which means going further without stopping or going faster with the same fuel. Being able to go higher and faster means getting from A to B quicker which is why rich people pay over the odds to use them.
---Edit ---
Looking at the comments, I clearly could have phrased that better.
An aircraft that 'can' do Mach .85 is by definition more aerodynamic that one that can only do .8, and an aircraft that can cruise at 45,000 ft can get better efficiency from its engines then one that can only cruise at 35,000ft (ie the same engine can go further or faster with the same fuel at 45,000ft then it can at 35,000ft), not to mention that at 45,000ft it can take a more direct path by being above the commercial traffic which also allows it to use its extra speed.  Combine these and an aircraft that can go higher and faster will get from A to B in less time then the same aircraft going lower or slower.
Business jets are not sold just on their Mach numbers, but the combination of their speed and cruising height.

Is this answer meaning to say, among other things, that business jet are not actually flying faster than airliners for the most part, they are just flying at higher altitude and therefore higher Mach for the same TAS?  I can't quite tell.

@quietflyer If that's what the answer says, it doesn't make sense - since above tropopause (36k feet), the temperature and therefore the speed of sound are constant. This means that a particular Mach no. always corresponds to a particular TAS; so an airliner flying at M0.8 at 36k feet travels at the same TAS as a corporate jet at M0.8 at 50k feet.

"an aircraft that can cruise at 45,000 ft can get better efficiency from its engines then one that can only cruise at 35,000ft" are you sure that the fuel consumption is lower at higher altitude?

@sophit - yes I am sure. Do you have any reason to doubt it?

No reason to doubt, but I have in mind that higher than sone 36'000ft the fuel consumption remains more or less constant but maybe I'm just remembering it wrong.

@sophit - consumption is roughly constant but air density drops so the same thrust has less resistance to overcome, resulting in more efficient use of fuel.

Ah, now I understand: is the drag that is less and not the engine's efficiency that is more.

@AdityaSharma: The altitude of the tropopause varies dramatically from below 30 kft to over 55 kft, depending on location.  It is not always (or even usually) at 36 kft.  Also, above the tropopause, the temperature rises with increasing altitude, rather than remaining constant.


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Why are jet aircraft never designed with a slower cruise speed?
All jet airliners have a cruise speed between Mach 0.82 and Mach 0.85.
At those speeds the aircraft are flying at their maximum subsonic speed. Any greater cruise speed would only be possible with major design changes.
My question is about fuel efficiency.
As drag increases with the square of speed, would a slower cruise speed lead to better fuel efficiency?
So why are jet aircraft never designed with a slower cruise speed?
If transonic phenomena didn't exist, would a faster speed be more fuel efficient?
Currently some airliners, after flying a 12-hour route, stay on ground for a long time before returning. Wouldn't a slow flight (if it is more fuel efficient) be better in this scenario? So not to waste time on ground?
On long-range flights, any fuel saving significantly increases the flight's profitability.

related: http://aviation.stackexchange.com/questions/3748/why-1st-and-2nd-class-but-not-fast-or-slow-airplanes

Only the long-range aircraft all cruise between M0.82 and M0.85. Short-range ones may cruise at less. For example A320 cruises at M0.78 (and yes, the reason is fuel efficiency).

The statement "All jet airliners have a cruise speed between M0.82 and M0.85" is absolutely false. Far more airliners cruise in the range .74-.80 mach than cruise in the range stated. For example, M0.82 is MMO for the 737; the 737 Next Gen cruises between roughly .77-.80, while the 737 Classics typically cruise around .74-.76.

In a single word, lift.  The speed moving forward is also what keeps the plane up, and compensating for lower speed would cause more drag.

The whole point of boarding a jet to get somewhere is to get there as quickly as possible - the Concorde had a market opportunity because it was so fast. Unfortunately, there are a whole lot of technical challenges with supersonic cruise which makes it unaffordable to the average ticket buyer. Add to that a few additional safety challenges and that is why the Concorde had a limited market all to itself and ultimately stopped flying. High subsonic is currently the optimum point for speed and affordability.

XB-70 Valkyrie had cruising speed M3+ because it used compression lift but of course it was not a commercial jet. :-)

Title refers to "jet aircraft" while question body narrows it to "jet airliners". PZL M-15 has cruise speed 140–165 km/h (76–89 knots, 87–103 mph).

There is indeed a benefit in flying more slowly, but the aircraft needs to be designed that way to profit from slower speeds. You can see that wing sweep has been slightly reduced in the more modern airliners, but this is also due to better airfoils with higher Mach drag rise onset.
But some things will also cost more when you slow down: You have to pay the crew for more hours, and the aircraft cannot be utilized as often. To transport the same number of passengers requires more aircraft when they fly at a lower speed. The cost versus speed relationship is not linear, but it should have a minimum somewhere below today's operating points. Aircraft with a slower design cruise speed can be lighter and smaller for the same payload, but for now airlines don't have a choice of slow or fast jets; all manufacturers try to design their long-range airplanes for at least Mach 0.83.
This has a lot to do with marketing: The aircraft with the higher speed will show a shorter travel time for the same distance, so it will show up first in the booking systems of travel agents. Of course, now you will argue that most people book on the internet and will try to get the lowest cost, regardless of travel time. True. But this is not the sort of customer the airlines are after. Their profit comes from people in First and Business class, and these still book predominantly via travel agents. Therefore, Airbus and Boeing try to market their planes as the ones which come first on the Amadeus and Sabre screens.
If you would single-mindedly optimize a configuration for the best fuel efficiency, you would arrive at something close to the MIT D8 design or Boeing's SUGAR study. Note that both are designed to fly quite a bit slower than current airliners at Mach 0.72. These designs are for a world in which fuel costs 200 USD per barrel or more, while the current designs are based on the expected prices at the time of their design start.
When fuel prices were low, the optimum speed for the best transport performance was indeed above Mach 1. The Vickers VC-10 of 1964 sported a maximum cruise Mach number of 0.886 and still holds the record for the fastest commercial Atlantic crossing. When Concorde was developed, the general consensus was that future air travel would be supersonic. Only the jump in oil prices after the 1973 oil crisis put those plans to rest. Large business jets today can cruise at up to Mach 0.935 because their owners are less concerned with saving money by flying more slowly. Technically, there is still room for cruising above Mach 0.85; it's economics that keeps airliners back.
If airplane companies were free to pick the optimum Mach number, they would today choose a speed between Mach 0.78 and 0.82 with fuel prices around 50 USD per barrel. Note that this is the design Mach number range for regional jets which fly short hops in which the increase in trip time from the reduced speed is insignificant. Also, especially in times of high fuel prices airlines operate their gear at lower speeds in order to conserve fuel. But if you follow this link, you will learn that airplane companies don't have that freedom.
Regarding drag:
Drag does not go up with the square of speed. When plotted over speed, the drag of an airplane will first go down, reach a minimum and only then go up. Airliners fly close to this minimum, and they fly high to shift this minimum to the highest possible flight Mach number. They do this by flying in less dense air, which requires to fly at an altitude around 30.000 - 40.000 ft. An additional benefit to flying this high is the lower air temperature which makes the engines more efficient.

@ymb1: Yes - and airliners could, too, if they were designed for it. Concorde and Tu-144 flew way higher again than the biz jets. Just give airliners bigger engines and a lower wing loading. The pressurization difference is negligible if you already fly at one quarter of sea level pressure. But no sane airliner company does this and no airline wants it (witness Boeing's Sonic Cruiser).

FYI, Jet A fuel at the self-serve pumps is between 3.50 and 7.00 dollars in California—not 50 USD. Airlines probably pay around 1.40.

@JScarry: The quoted fuel price is $50 per BARREL, which is 42 US gallons (if I can trust Google).  Which is close to your $1.40/gal.

Short answer: No one wants a slow plane.
Long Answer: While we can talk about engine efficiency all day, we must remember that planes are designed more for missions than science in many regards. If we take the fact that generally speaking planes are built to move things (people/cargo) far and fast there is not much of a use case present for a slow but efficient plane. While this would present savings in terms of cost to the passenger, most people are willing to pay for the speed a less efficient plane offers. Perhaps the best example of this is the inefficient and expensive Concorde which many people were willing to pay large sums of money to fly just to save a matter of hours.

On long range flights, any fuel saving significantly increases the flight's profitability.

This is a bit of a blanket statement and I'm not sure "significantly" is an appropriate word. While fuel savings can increase profitability from a fuel standpoint, you do so at the cost of other expenses. For example most plane parts are serviced based on flight hours, so for a given airframe/number of flights, you will increase your spend on inspections since the plane is presumably spending more time in the air to move a given load. You also have the issues of paying the crew which can cut into costs. Considering long flights (where crew cycle on 8 hour rotations) you may require a full extra crew if time is significantly increased.
I don't have any hard stats on this (but I'll look for some) I would assume there is a safety risk involved with simply spending more time in the air. If we look at accidents per flight hour reason stands that the more time spent in the air the higher chance of an accident, but this is a bit of an educated guess.

Regarding your accidents per flight hour, as soon as I began to read that bit, I went to the opposite conclusion. Since the risk on an accident is so low, even with more planes being in the air I'd think the numbers of hours would climb much faster and drive the stats down.

I know I'm stupid. I like flying as passenger and therefore I would probably fly only once with the Concorde (yeah mach 2!), on every other occasion I would prefer regular jet around 0.8 - 0.95. I enjoy flying and traveling to much! Faster planes, bad. Slower planes, bad too. "Beaming" would steal my journey...

@HorusKol "During its time in service, Concorde’s profitability figures weren’t released by BA but it is understood that the service wasn’t a profitable enterprise." http://www.telegraph.co.uk/luxury/travel/98151/concorde-40-fascinating-facts.html

Although ""In the past, the Concorde was profitable," Mr. Noyes said, but it was losing money after it was returned to service following an Air France crash in 2000 that killed 113 people. A spokesman for Air France, Jean-Jacques de Saint Andrieu, said that his airline's Concorde operation was profitable "in some years." http://www.nytimes.com/2003/10/24/nyregion/24CND-AIR.html

A lot of that is actually speculation, the Concorde was profitable according to this interview with one of the chief pilots since its design and development was undercut by the government it cost BA/AF little to actually acquire the plane and due to some interesting ticket hike ideas became quite a money maker. You should listen to the full interview there, lots of interesting stuff.

Short answer - at current fuel prices, it's the most economical speed from a cost/revenue perspective.
The two biggest costs of operating an airliner are fuel, which would be reduced by a slower and more efficient aircraft, and airframe utilisation - The cost of the plane divided by how many ticket sales you get out of it over its lifespan, which, would be reduced by flying faster. Additionally flight crew are paid by the hour. Currently M0.85 or a bit less is a sweet-spot in these curves as going transonic causes fuel use to rise more aggressively with speed, but designs for slower and more efficient aircraft (even higher bypass engines; less swept high aspect ratio laminar flow wings) haven't got past concepts because their overall profitability would be despite lower fuel costs, especially when you consider that the tickets would have to be sold for less to make them compete with faster flights on current planes.

In addition to the other, more technical answers, there is another reason why aircraft aren't designed to spend more in the air rather than sitting idle at either end of the route - pilots and air crew have a maximum number of hours they are allowed to be "on duty".
Under FAA rules governing Duty Limitations, a pilot can spend no more than between 9 and 14 hours on duty, depending on certain factors such as whether there are other pilots scheduled on the same flight.  In addition to that, a pilot must have at least 10 hours of uninterrupted rest time between their duty periods.
So, the slower the aircraft flies, the longer it is in the air - which means that more pilots are required to cover the same flight.
For example, if an aircraft takes 9 hours to fly a route, then under FAA rules the same crew can take a 10 hour rest break at the other end and then return the aircraft.
But if the aircraft takes 10 hours to fly the route, the FAA requires another crew to ensure safety, but both crews must take their mandatory 10 hour rest break at the end of the flight, and then both crews can fly the same route back.
You haven't saved anything, but you are paying for extra crew members.
Airlines and aircraft manufacturers are in a constant push-and-pull struggle to get the most out of their aircraft at any one time, while safety bodies such as the FAA, CAA, EASA et al are constantly ensuring that the routes the airlines fly are safe.
http://work.chron.com/duty-limitations-faa-pilot-17646.html

First you must realize that mach number is not a speed, but a ratio relating to speed and air temperature. The higher the altitude, the less dense the air. The crux of understanding the whole speed vs efficiency situation is in the difference between the different airspeed types and how they affect aerodynamics and the speed of sound. So it isn't the faster you go but the higher you go that increases efficiency.
Different types of airspeed
The ground speed is related to the true airspeed (TAS), which is how fast you are moving through the volume of air, regardless of the density. Whereas lift and drag are determined by indicated air speed (IAS), which is basically how fast you are going through the mass of air. So while maintaining the same IAS, as you ascend the TAS is higher, thus a faster ground speed. You must maintain IAS above stall, so, to get the most TAS out of it (thus better ground speed) you want to fly in the lowest air density you can.
Speed of sound
The plane is limited, as you note, by the local speed of sound (LSOS), which is affected by the TAS and the temperature. The higher you go, the colder it gets (to a certain point) and thus the lower the LSOS. So at a given IAS the lower the air temperature the higher the mach number (closer to the speed of sound).
Finding the sweet spot
Since lift is related to IAS the TAS is pretty much irrelevant to the wings as long as you stay below the point where LSOS issues arise. You must maintain IAS so that lift is equal to gravity, so it doesn't change at the same altitude. As altitude increases and temperature decreases the TAS goes up and the LSOS comes down. The mach number is the ratio between these two. So there is a point where they converge and you can't go any higher because your TAS will exceed the aircraft mach limit. That's the so-called "coffin corner." If you slow down you stall, if you speed up you exceed the mach limit. The sweet spot for efficiency will be in this area.

Part 1 of answer:  Maximum efficiency (minimum thrust/fuel requirement) occurs at the speed of maximum lift to drag ratio. It’s determined mainly by the wings, for jets engine efficiency as a function of speed also matters.
Sonic drag is another factor, most aircraft need very hish power to accelerate through Mach 1. Aircraft designed for supersonic flight experience decreasing drag above Mach 1 until dynamic pressure, proportional to the square of speed, overwhelms other factors.
A notable case was the Lockheed F-104 with the -19 model of its J79 jet engine. With that engine its fuel efficiency as a function of distance traveled improved above Mach 1, at least to Mach 2 — One pilot report said fuel burn for a given distance traveled was best at about Mach 2. That pilot wrote about returning from Texas to Florida doing Mach 2 at 73,000 feet, with the cruise taking an hour.

To understand why an aircraft will be more efficient when it flies faster you must realise that the aircraft gives the air that it passes through a downward velocity. The change in the momentum of this air is what supports the mass of the aircraft. Every kilogram of the aircraft's mass will produce a downward force of about 10 newtons. This must be balanced by a change of momentum of 10 kgms-1 every second.
Now, although the change of momentum of a given mass is proportional to the change of velocity, the energy required to produce that change is proportional to the velocity squared. (In SI units the energy is (mv2)/2.) So the 1 kg mass could be supported for 1 second by accelerating 1 kg of air to 10 ms-1 or by accelerating 2 kg of air to 5 ms-1. Both will support the same mass but the first requires 50 J of energy and the second 25 J. Thus by doubling the mass of air that supports the aircraft the power required to do so has been halved.
So to get the best possible efficiency an aircraft should displace the greatest possible mass of air. However, it can only affect the air that it passes through, so in order to move a greater mas of air it must either increase its wing span (which is why high performance gliders have very long wings) or travel faster. So if all other factors were to remain the same then if an aircraft were to double its speed it would require half as much power to keep it up. In addition it would travel twice the distance, so the energy, and hence fuel, needed to travel a unit distance would be reduced by a factor of 4.


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What are the advantages of twin-fuselage aircraft?
During WWII, there were a number of twin-fuselage aircraft (notably bombers but some fighters). They were made by apparently combining two existing single-fuselage aircraft bodies together:

Twin-fuselages seemed to disappear after the 1960s but  recently, Virgin Galactic has come out with a twin-fuselage aircraft, the WhiteKnightTwo.

I understand the Virgin design as the space module (SpaceShipTwo) is designed to fit between the two fuselages. But aside from that special case, are there any advantages to twin-fuselage aircraft?
Was their design and introduction in the 1930s-1950s merely a strange historical accident (and if so, what was the reason d'etre then) or do they currently or potentially have advantages over single-fuselages such that we might see their return?

would you mind credit the source of your pics. Tks :)

Wikimedia commons.

Depending on the design, one of three advantages made designers choose a twin-boom layout:

No structure directly aft of the fuselage, which helped to integrate the propulsion, or improved access to the cargo area, or improved pilot vision,
Smaller overall surface area for two-engine aircraft, or
Shorter development time for high-performance aircraft.

There are two groups of twin boom designs: Those which were designed that way from the outset, and those which were later derived from single-fuselage designs. You could add a third group, those with two distinct fuselages and an offset pilot position, but an offset cockpit could also be a result of other design considerations. Examples are the Junkers 322 or the Blohm&Voss BV 141, both single-fuselage designs with an offset cockpit.
The first group comprises aircraft like the D.H. 2, the Saab 21 (picture below, source) or the Gotha Go-242. Here the designers wanted to create space for the propeller or a loading ramp. Also, some early jet aircraft used a two-boom layout to keep the tail structure free of the jet blast. The advantage is obvious: No structure at the rear fuselage.

The second group arose due to the performance requirements of modern, all-metal, stressed-skin designs with cantilevered wings. Remember that at the beginning of the 1930s fighters were still biplanes (Hawker Fury, Heinkel He-51), and new designs like the Caudron 460 or the Messerschmitt Bf-108 showed that sleek aerodynamics and high wing loading were the way ahead. However, unlocking the performance potential possible with these designs required much more powerful engines. The engine manufacturers could increase power output only slowly, so a number of designs (Lockheed P-38, Messerschmitt Me-110, Bristol Beaufighter, Northrop P-61) tried to use two engines. Since there were two engines already with a big space requirement for the turbocharger, Lockheed decided to use a two-tailboom design. Here the advantage was a smaller overall surface area to house two engines, their coolers and turbochargers, and the pilot (picture below, source).

During the war, again the requirement for more powerful aircraft arose, and to avoid the delay of a fully new design, existing aircraft were combined, shortening the development time greatly. The first of those was the Heinkel He-111 Z (for Zwilling = Twin), which added a fifth engine to be able to pull the giant Me-321 transport glider. Joining two powerful aircraft was proposed for the Messerschmitt Bf-109 (Bf-109 Z), the Messerschmitt  Me-309 (Me 609), the Dornier Do 335 (Do 635), the Savoia-Marchetti SM.92 and the North American P-51, the only one of those to result in an operational aircraft, the F-82. This resulted in an agile aircraft with high speed and high range, which was very useful for escort or reconnaissance tasks. However, only few of those were used operationally, because soon jet aircraft filled that need in a better way.
The main disadvantages of the twin fuselage designs with their offset pilot position is the high radius of roll inertia and the vertical acceleration suffered by the pilot during rolling maneuvers. Especially for a fighter, a fast roll response is the key to success in aerial combat, and a twin fuselage will impose a penalty not suffered by a two-engined aircraft with a central fuselage. This is compounded by the pilot suffering additional vertical acceleration when rolling, which again will reduce the likely roll agility of the twin-fuselage aircraft.
Might we see more twin-boom or twin-fuselage designs in the future? Probably only for special purposes, like the example you have given yourself. Another might be the Kensgaila VK-7, a Lithuanian airfoil testbed which combines two Blanik gliders with a special, exchangeable center section for testing new airfoils under realistic conditions. This one is not even on Wikipedia's list of twin-fuselage aircraft!
Earlier twin-boom transport aircraft like the Fairchild C-119 or the Nord Noratlas have given way to conventional layouts with a raised tail to accommodate the rear ramp. Twin-boom and twin-fuselage designs are less stiff and more prone to flutter than conventional configurations, so the conventional design prevailed and will do so in future as well.

Your examples are mostly twin tail-boom aircraft that have the pilot in a central fuselage. I am primarily interested in examples where there are two fuselages but your examples have also piqued my interest in twin booms. Can you edit your answer to differentiate between twin fuselage and twin boom?

@RoboKaren: I tried, but there are very few twin-fuselage designs. The disadvantage of the offset pilot position is most obvious in maneuvering, and all twin-boom and twin-fuselage designs are only good for low speeds when flutter is less prevalent.

As paul mentioned, the dual-fuselage fighters of WWII were primarily just ways to build a longer-range fighter much more quickly than starting with a completely new airframe design.
The F-82 specifically was designed to escort long-range bombers on bombing missions over Japan from U.S. bases in the Pacific (the Solomon Islands or the Philippines, for example.) However, the war ended before it entered service.
The Germans also designed the twin-fuselage He 111Z as a tug to tow troop-drop gliders in WWII. Again, this was much faster to get to production than designing a new aircraft. Obviously, time-to-production is a very big consideration when you're in the middle of an ongoing war and need some specific capability for the war. In such cases, earlier service entry may override a more elegant design.
Another use that Wikipedia mentions for dual-fuselage aircraft was for early seaplanes. In these cases, the dual fuselages added stability on the water, similar to the way pontoons or outriggers are used, especially for flying boat designs where the aircraft uses the bottom of the fuselage as a hull in the water rather than using separate floats.
Finally, as you mentioned, the most recent uses of dual-fuselage aircraft have been for space launch vehicle carriers, such as the White Knight Two and the ridiculously-massive Scaled Composites Roc built for Stratolaunch Systems. The reasoning here is that you obviously want the payload that you're going to be dropping mid-flight to be along the centerline of the aircraft for CG reasons. When said payload is something the size of a space launch vehicle, a dual-fuselage design allows you to leave a really big space to mount the launch vehicle while still keeping the weight balanced laterally.
Scaled Composites Roc carrier aircraft:

Image source: Wikipedia

Since you have the "Roc" pictured: Twin fuselage also allows mass distribution to be closer to lift distribution when flying and to the carriage when grounded. This reduces bending stresses (from lift and gravity) on the wings for aircraft with extremely long wingspans but low need for aerial agility. This is probably a factor for space launch vehicle carriers, probably not for older examples.

The F-82 in the top picture is the result of a wartime need for a long-range fighter-bomber. It takes a long time to design and tool up for a new aircraft, it takes rather less time to use your existing tools and rearrange the pieces you already have.
Long range means more fuel, that means more power, and in 1945 that meant a second engine. So, lets take two of those, build the center part with existing tools and we're done.
A more detailed explanation can be found on Wikipedia.

Could you explain better why long range in that period was a second engine? Were there technological limitations for having bigger engines?

@TrebiaProject.: I think he's saying that they already had the tooling to build the smaller engines, mounts, etc.

in 1945 we were basically at the practical limit for single-engine piston-prop aircraft (note that the 27L Merlin engine has more power than a PT-6 turbine). Two Merlins in series would have been buildable but you now need a bigger prop, which means higher landing gear, which means a new fuselage and now we are at the "all-new aircraft" stage, and there's a war on so delivery by next Thursday would be appreciated.

There's an entirely new type of twin fuselage plane being designed. It is really something halfway between a flying wing and a twin fuselage, so if this counts as twin fuselage for your purpose you'll need to decide for yourself.

It's called the Flying-V, and the reason for the design is to get a lot higher fuel efficiency. It's basically taking the aerodynamic advantages of a flying wing and modifying that idea to be practical for passenger transport. The designers claim it will use 20% less fuel than an Airbus A350.

I'm an absolute bystander without much (if any) knowledge of aviation, but this just seems like another one of the "great new - but not really - ideas" that pop up every year in headlines and then you never ever hear from them again because they severely underestimated some practical problem or were there just to impress gullible investors to "gib cash"

It's a research project from a prestigious technical university in The Netherlands. And that flying wings are more efficient than conventional designs is also an established fact, so I expect those technicalities to be correct. Of course from a research project to actual production is a long long road and will require a lot of $$$, so I wouldn't be very surprised if we never hear from the concept again. On the other hand, there might be some aircraft builder who's willing to invest in serious efficiency gains, so if this really is the best way to improve efficiency it might work out.

I have no confidence in this design at all. It was a bright idea from a student. My guess is it was initially modelled with 2D flow software, but as Handley Page confirmed with the HP.115, the flow over a wing this swept is essentially sideways and that turns out to be far less efficient at subsonic speeds. Now a university has taken it over, I fully expect them to rediscover this when they apply 3D flow analysis.

@GuyInchbald Originally it was a 'bright student idea', but the project is much further now. The shape of the plane was optimized using CFD and structural simulations, there have been windtunnel tests and there is a flying scale model (the scale model flight was reported in Dutch media, which is how I know). (See project website and e.g. this article.) Professors from several chairs are involved, and some students graduated on work on the project. It is sponsored by Royal Dutch Airlines and Airbus.

@JanKanis The flying model did not investigate cruise characteristics. Concorde had a broadly comparable leading edge sweep and was inefficient below Mach 1. Even the Handley Page Sycamore slewed-wing SST reserved such sharp sweep angles for supersonic speeds. Do I believe the experience of real designers or enthusiastic claims arising from the use of low-cost software pushed to extreme scenarios? We shall just have to wait and see.


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Is "secondary radar" a misnomer?
Radar is based on the principle of transmitting radio waves and measuring the length of time-of-flight, direction, and frequency shift of the returning reflections or echoes.
In aviation, we call this "primary radar" and we use the term "secondary radar" to denote the ATC/transponder system.
Secondary radar uses a transmitter that sends pulses that the transponder picks up and then sends coded pulses of its own back. But these are not based on reflections off the airframe.
Why then call the ATC/transponder system "secondary radar" when the principle is very different from what we might call "real" (aka primary) radar?  Or does the "secondary" here meaning "in addition to" rather than "another form of"?

It still uses radio waves to detect aircraft and find the distance to them, right?

Yeah, but by that definition the ATC contacting the plane by radio and verbally asking the pilot for a GPS position readout would also be radar. Which is essentially what SSR is, but automated (and not as detailed).

SSR doesn't ask the plane for its position, though; it computes distance and azimuth from timing, just like primary radar.

@cpast is correct. Only the altitude and 'squawk' code are encoded in the response (in Mode C, at least... a bit more goes into Mode S responses, IIRC.) In addition to timing, though, actual antenna response differences are measured to help determine bearing more precisely than could be done with timing alone. The timing could be faked (by delaying the response,) but faking antenna response differences would be really hard to pull off reliably.

So I don't know the "real" answer to this so I'm not sure which of the answers to select. Tyler's seems just as good as Greg's -- but don't know why there is currently a 9:1 in favor of Greg's. Is this because Greg is more correct (why?) or simply because he was the first to post? Advice on which to select is appreciated.

Without going through the history, better terminology would be the primary is a passive system, relying on a reflected return from the aircraft.  The secondary system is an active system, which broadcasts a reply in response to an interrogation. Azimuth and time of "flight" are used to determine the location of the transponder responding to the interrogation.  Newer secondary modes do not require an interrogator and self-report location.  Probably not really secondary radar, but the function is similar.  All three methods are independent.

@mongo Those terms are switched. Primary Surveillance Radar is an active radar system - bounces signals off of aircraft and listens to the reflection. Passive radar listens to reflections from an unintentional source (eg radio towers). Secondary Surveillance Radar isn't quite either (sort of active because it sends out its own signal, but doesn't use actual reflections).

The terminology is a combination of history and "trustworthiness" (for lack of a better term).
Historically, primary radar was invented first. So it makes sense that this would be called "primary" and another scheme invented later would be called "secondary".
ATC systems like to ensure that the data they are working with is trustworthy. Primary radar systems are based only on the reflected radar signals. If you get a reflection, you know there is something there, whether or not it has a transponder (or indeed, whether or not it is an aircraft). This is a certainly a key point in military radar systems, and ATC radar was derived directly from military radar.
With secondary radar (transponder) operation, there is no guarantee that the transponder isn't lying (whether deliberately or unintentionally). A faulty transponder could certainly report false locations to the ATC system. Or, you can probably imagine scenarios where a false location might be deliberately returned.
ATC systems take both the primary and secondary location information for an aircraft, and correlate them to represent one aircraft in the sky. They don't report the exact same physical location, so there is a degree of fuzziness in the system to account for that. If the two reported locations drift too far apart, you get an uncorrelated track. Sometimes, if two aircraft cross each other's track in certain ways, the ATC system can get confused and associate the wrong primary radar blip with the wrong transponder report. The controllers have procedures to deal with and correct these kinds of problems.
So, primary radar is primary because it (a) came first, and (b) is the most trustworthy source of aircraft location information. Secondary "radar" (which, as you point out, is not really radar) is called such because it is integrated within the primary radar system.
More generally, the various technologies for locating and identifying aircraft in airspace are collectively called "surveillance". Examples of ATC surveillance are primary radar, transponders, multilateration, ACARS position reports, and other technologies.

Well, "not really radar" depends on exactly how you define radar. Secondary radar is RAdio Detection And Ranging. It does not, however, use reflections, like primary radar. It relies on the transponder to tell the correct altitude and timing of the response (along with whether or not there is a response) to determine distance and bearing.

The reason why it is called "secondary" radar is because it is in addition to the primary existing radar system, the surveillance system.
In other words even with no transponders the radar system would still exist because large airports and regional zones have air surveillance radars that monitor airspace and show contacts. These contacts include everything that reflects radar energy; not just planes, but birds, even clouds of dust, etc. The is the primary radar system.
The transponders leverage this existing system by transmitting signals these radars can receive and interpret. This allows us to put a bright dot on the screen (and other information like the code) because we know it is an aircraft and it is not, for example, a bird. Because this identification and localization system only works with transponders it is considered a secondary system. Since the primary radar always works, even if the aircraft does not have a transponder, it is the primary system. It is also primary in the sense that originally there were only surveillance radars; the interrogating radars that receive transponder signals were developed afterwards.
To further illustrate this, refer to this image of a typical ATCRBS antenna:

The larger curved antenna is the PRIMARY antenna, it detects aircraft (and everything else, birds, etc) uncooperatively. The smaller, ladder-like antenna on top of the primary is the SECONDARY antenna. This is what is used to detect the transponder's signal. The information from the secondary is then (traditionally) overlaid on the primary's display (although nowadays they are often digitally combined).

I'm not sure what you are trying to say, nor that I agree with you. There is almost no commonality between primary and secondary RADAR. Switch off the transponder and you will disappear on secondary RADAR. There is no transmitted/reflected pulse. It does not "repurpose" anything. It is a completely different concept and certainly not "built on top" of a primary system.

@Simon I have changed my language a little bit to make it clearer and added more explanation with a photo. The secondary is on "top" of the primary in the sense that the data from the secondary is overlaid on the display of the primary, and because the PSR system was the original system and the SSR was added onto it later.

@Simon Also the SSR antenna is literally physically on top of the PSR, as you can see in the photo.

@Simon Secondary radar still determines position the same basic way as primary radar, just with a transmitter reply instead of a reflection.

@cpast Yes of course, but in a very different manner.  My comments were to the original answer which seemed to imply that SSR was some kind of extension to primary RADAR.  Tylers' edits have done a good job of clarifying that.

@Simon: From the controllers' perspective, it is an extension to primary radar, one that provides them with considerably more information than primary radar does.


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Why did my business class seat have a three point shoulder harness?
I recently rode a short haul international flight on Asiana Airlines AAR111 from KIX to ICN. I was surprised that my business class seat had a three-point shoulder harness given that we’ve discussed many times on this SE why shoulder harnesses aren’t provided to customers.
Interestingly, on the flight back I flew Korean on the same route and KAL [business class] seats don’t have three points, only standard seat belts.
Is there a reason AAR111’s business class has three point harnesses? Why just that plane and not similarly configured other flights?

Related: Why are passengers not given the option of shoulder harnesses?: https://aviation.stackexchange.com/questions/29211/why-are-passengers-not-given-the-option-of-shoulder-harnesses/29220#29220

It's always a regulation. If there is any safety equipment on an aircraft, a reg put it there. In this case it is FAR 25.562 Amendment 25-64, the 16g seat impact requirement.
This reg included a "head strike envelope" provision that manufacturers say is so complex that it was impossible to satisfy. This was with a lap belt only. The FAA "relented" by allowing addition of a shoulder belt as a substitute level of protection, that did not need all the time consuming testing.
You would have to believe the manufacturers were telling the truth, because they will do almost anything to keep weight out of the airplane.

On some seats, I believe the regulatory requirements are met with airbags instead of shoulder belts. You sometimes see this in bulkhead rows of economy if the cabin layout doesn't meet the requirements to prevent head strikes. I've seen this on business seats as well, in lieu of a shoulder belt.